On 06/08/2024 21:29, Peter Xu wrote: > On Tue, Aug 06, 2024 at 06:37:55PM +0200, David Hildenbrand wrote: >> On 06.08.24 17:15, Ryan Roberts wrote: >>> Hi Peter, David, > > Hi, Ryan, > >>> >>> syzkaller has found an issue (at least on arm64, but I suspect it will be >>> visible on x86_64 too) that triggers the following warning: > > This is true. I can easily reproduce.. > >>> >>> [ 2291.836518] ------------[ cut here ]------------ >>> [ 2291.836528] WARNING: CPU: 3 PID: 9056 at mm/page_table_check.c:207 __page_table_check_ptes_set+0x22c/0x248 >>> [ 2291.836541] Modules linked in: >>> [ 2291.836549] CPU: 3 UID: 1000 PID: 9056 Comm: bug Tainted: G W 6.11.0-rc2-dirty #2 >>> [ 2291.836554] Tainted: [W]=WARN >>> [ 2291.836557] Hardware name: linux,dummy-virt (DT) >>> [ 2291.836559] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) >>> [ 2291.836564] pc : __page_table_check_ptes_set+0x22c/0x248 >>> [ 2291.836568] lr : ptep_modify_prot_commit+0x24c/0x2b0 >>> [ 2291.836573] sp : ffff80008ca6ba20 >>> [ 2291.836575] x29: ffff80008ca6ba20 x28: ffff186392d1eb00 x27: 0000000020ffd000 >>> [ 2291.836598] x26: 0010000000000001 x25: 0000000000000001 x24: 0000000000000000 >>> [ 2291.836605] x23: 04e800018c738f43 x22: 0000000000000001 x21: ffff1863824163c0 >>> [ 2291.836612] x20: 04e800018c738f43 x19: 04e800018c738f43 x18: 0000fffff7f87fff >>> [ 2291.836619] x17: 0000000000000000 x16: 1fffe30c748d22a1 x15: 0060000000000fc3 >>> [ 2291.836625] x14: 0000000000000000 x13: 0000000020ffd000 x12: 0000fffff7f87fff >>> [ 2291.836631] x11: 0000000020ffd000 x10: 0000000000000000 x9 : ffffbcab99e3ab84 >>> [ 2291.836638] x8 : ffff186382b8f000 x7 : 0000000020ffe000 x6 : 0000000020ffd000 >>> [ 2291.836644] x5 : ffff186392d1eb00 x4 : 04e800018c738f43 x3 : 0000000000000001 >>> [ 2291.836650] x2 : 04e800018c738f43 x1 : ffff18639fe01fe8 x0 : ffffbcab9ce56780 >>> [ 2291.836657] Call trace: >>> [ 2291.836659] __page_table_check_ptes_set+0x22c/0x248 >>> [ 2291.836664] ptep_modify_prot_commit+0x24c/0x2b0 >>> [ 2291.836667] change_protection+0x8a0/0x1100 >>> [ 2291.836672] mprotect_fixup+0x124/0x2d0 >>> [ 2291.836675] do_mprotect_pkey.constprop.0+0x29c/0x460 >>> [ 2291.836679] __arm64_sys_mprotect+0x24/0xf8 >>> [ 2291.836682] invoke_syscall+0x50/0x120 >>> [ 2291.836690] el0_svc_common.constprop.0+0x48/0xf0 >>> [ 2291.836694] do_el0_svc+0x24/0x38 >>> [ 2291.836699] el0_svc+0x34/0xe0 >>> [ 2291.836705] el0t_64_sync_handler+0x100/0x130 >>> [ 2291.836709] el0t_64_sync+0x190/0x198 >>> [ 2291.836713] ---[ end trace 0000000000000000 ]--- >>> >>> The generated program (see below) mmaps a 16M region (RWX). It then mlocks all >>> current and future memory. >>> >>> Next, it registers 12K (3 pages) for use with UFFD-WP, and marks 4 pages >>> UFFD-WP'ed. This returns ENOENT because we only registered 3 pages, but those 3 >>> pages are still UFFD-WP'ed in their PTE, so this error is not relavent to the >>> bug. At this point, there is a single VMA covering the 12K, with VM_UFFD_WP set, >>> amongst other flags: >>> >>> 20ffb000-20ffe000 rwxp 00000000 00:00 0 >>> Size: 12 kB >>> KernelPageSize: 4 kB >>> MMUPageSize: 4 kB >>> Rss: 12 kB >>> Pss: 12 kB >>> Pss_Dirty: 12 kB >>> Shared_Clean: 0 kB >>> Shared_Dirty: 0 kB >>> Private_Clean: 0 kB >>> Private_Dirty: 12 kB >>> Referenced: 12 kB >>> Anonymous: 12 kB >>> KSM: 0 kB >>> LazyFree: 0 kB >>> AnonHugePages: 0 kB >>> ShmemPmdMapped: 0 kB >>> FilePmdMapped: 0 kB >>> Shared_Hugetlb: 0 kB >>> Private_Hugetlb: 0 kB >>> Swap: 0 kB >>> SwapPss: 0 kB >>> Locked: 12 kB >>> THPeligible: 0 >>> VmFlags: rd wr ex mr mw me uw lo ac >>> >>> Next we mremap the first page to the address where the last page was previously >>> mapped, with MREMAP_DONTUNMAP. This leads to 2 VMAs, but the new one doesn't >>> have VM_UFFD_WP set (Note also that the original VMA no longer has VM_LOCKED >>> which seems wrong to me, but I'll ignore that for now): >>> >>> 20ffb000-20ffd000 rwxp 00000000 00:00 0 >>> Size: 8 kB >>> KernelPageSize: 4 kB >>> MMUPageSize: 4 kB >>> Rss: 4 kB >>> Pss: 4 kB >>> Pss_Dirty: 4 kB >>> Shared_Clean: 0 kB >>> Shared_Dirty: 0 kB >>> Private_Clean: 0 kB >>> Private_Dirty: 4 kB >>> Referenced: 4 kB >>> Anonymous: 4 kB >>> KSM: 0 kB >>> LazyFree: 0 kB >>> AnonHugePages: 0 kB >>> ShmemPmdMapped: 0 kB >>> FilePmdMapped: 0 kB >>> Shared_Hugetlb: 0 kB >>> Private_Hugetlb: 0 kB >>> Swap: 0 kB >>> SwapPss: 0 kB >>> Locked: 0 kB >>> THPeligible: 0 >>> VmFlags: rd wr ex mr mw me uw ac >>> 20ffd000-20ffe000 rwxp 00000000 00:00 0 >>> Size: 4 kB >>> KernelPageSize: 4 kB >>> MMUPageSize: 4 kB >>> Rss: 4 kB >>> Pss: 4 kB >>> Pss_Dirty: 4 kB >>> Shared_Clean: 0 kB >>> Shared_Dirty: 0 kB >>> Private_Clean: 0 kB >>> Private_Dirty: 4 kB >>> Referenced: 4 kB >>> Anonymous: 4 kB >>> KSM: 0 kB >>> LazyFree: 0 kB >>> AnonHugePages: 0 kB >>> ShmemPmdMapped: 0 kB >>> FilePmdMapped: 0 kB >>> Shared_Hugetlb: 0 kB >>> Private_Hugetlb: 0 kB >>> Swap: 0 kB >>> SwapPss: 0 kB >>> Locked: 4 kB >>> THPeligible: 0 >>> VmFlags: rd wr ex mr mw me lo ac >>> >>> Finally we try to mprotect that last 4K region to remove X, and we get the >>> warning saying the PTE has both the UFFD-WP and WRITE bits set. >>> >>> I'm guessing this is because the VM_UFFD_WP flag got spuriously dropped when >>> creating the final 4K VMA and so mprotect's can_change_pte_writable() check >>> incorrectly allowed the pte to be marked writable. But the mremap man page is >>> not very clear on the semantics when interacting with uffd regions; perhaps >>> uffd-wp bit should have been cleared when mremapping the ptes? >>> >>> I'm hoping you can advice on the expected semantics and we can figure out how to >>> solve this? >>> >>> >>> The reproducer is as follows (with a few annotations added by me): >>> >>> """ >>> // autogenerated by syzkaller (https://github.com/google/syzkaller) >>> >>> #define _GNU_SOURCE >>> >>> #include <endian.h> >>> #include <stdint.h> >>> #include <stdio.h> >>> #include <stdlib.h> >>> #include <string.h> >>> #include <sys/syscall.h> >>> #include <sys/types.h> >>> #include <unistd.h> >>> >>> #ifndef __NR_ioctl >>> #define __NR_ioctl 29 >>> #endif >>> #ifndef __NR_mlockall >>> #define __NR_mlockall 230 >>> #endif >>> #ifndef __NR_mmap >>> #define __NR_mmap 222 >>> #endif >>> #ifndef __NR_mprotect >>> #define __NR_mprotect 226 >>> #endif >>> #ifndef __NR_mremap >>> #define __NR_mremap 216 >>> #endif >>> #ifndef __NR_userfaultfd >>> #define __NR_userfaultfd 282 >>> #endif >>> >>> uint64_t r[1] = {0xffffffffffffffff}; >>> >>> int main(void) >>> { >>> intptr_t res = 0; >>> >>> syscall(__NR_mmap, /*addr=*/0x1ffff000ul, /*len=*/0x1000ul, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/-1, /*offset=*/0ul); >>> syscall(__NR_mmap, /*addr=*/0x20000000ul, /*len=*/0x1000000ul, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/-1, /*offset=*/0ul); >>> syscall(__NR_mmap, /*addr=*/0x21000000ul, /*len=*/0x1000ul, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/-1, /*offset=*/0ul); >>> >>> write(1, "executing program\n", sizeof("executing program\n") - 1); >>> >>> // userfaultfd(UFFD_USER_MODE_ONLY) = 3 >>> res = syscall(__NR_userfaultfd, /*flags=UFFD_USER_MODE_ONLY*/1ul); >>> if (res != -1) >>> r[0] = res; >>> >>> // ioctl(3, UFFDIO_API, {api=0xaa, features=0 => features=UFFD_FEATURE_PAGEFAULT_FLAG_WP|UFFD_FEATURE_EVENT_FORK|UFFD_FEATURE_EVENT_REMAP|UFFD_FEATURE_EVENT_REMOVE|UFFD_FEATURE_MISSING_HUGETLBFS|UFFD_FEATURE_MISSING_SHMEM|UFFD_FEATURE_EVENT_UNMAP|UFFD_FEATURE_SIGBUS|UFFD_FEATURE_THREAD_ID|UFFD_FEATURE_MINOR_HUGETLBFS|UFFD_FEATURE_MINOR_SHMEM|0x1f800, ioctls=1<<_UFFDIO_REGISTER|1<<_UFFDIO_UNREGISTER|1<<_UFFDIO_API}) = 0 >>> *(uint64_t*)0x20000000 = 0xaa; >>> *(uint64_t*)0x20000008 = 0; >>> *(uint64_t*)0x20000010 = 0; >>> syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc018aa3f, /*arg=*/0x20000000ul); >>> >>> syscall(__NR_mlockall, /*flags=MCL_FUTURE|MCL_CURRENT*/3ul); >>> >>> // ioctl(3, UFFDIO_REGISTER, {range={start=0x20ffb000, len=0x3000}, mode=UFFDIO_REGISTER_MODE_WP, ioctls=1<<_UFFDIO_WAKE|1<<_UFFDIO_COPY|1<<_UFFDIO_ZEROPAGE|1<<_UFFDIO_WRITEPROTECT|0x120}) = 0 >>> *(uint64_t*)0x20000180 = 0x20ffb000; >>> *(uint64_t*)0x20000188 = 0x3000; >>> *(uint64_t*)0x20000190 = 2; >>> *(uint64_t*)0x20000198 = 0; >>> syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc020aa00, /*arg=*/0x20000180ul); >>> >>> // ioctl(3, UFFDIO_WRITEPROTECT, 0x20000080) = -1 ENOENT (No such file or directory) >>> *(uint64_t*)0x20000080 = 0x20ffb000; >>> *(uint64_t*)0x20000088 = 0x4000; >>> *(uint64_t*)0x20000090 = 1; >>> syscall(__NR_ioctl, /*fd=*/r[0], /*cmd=*/0xc018aa06, /*arg=*/0x20000080ul); >>> >>> syscall(__NR_mremap, /*addr=*/0x20ffb000ul, /*len=*/0x1000ul, /*newlen=*/0x1000ul, /*flags=MREMAP_DONTUNMAP|MREMAP_FIXED|MREMAP_MAYMOVE*/7ul, /*newaddr=*/0x20ffd000ul); >>> syscall(__NR_mprotect, /*addr=*/0x20ffd000ul, /*len=*/0x1000ul, /*prot=PROT_WRITE|PROT_READ*/3ul); >>> >>> return 0; >>> } >>> """ >>> >>> I'd appreciate any thoughts you may have! >> >> Interesting. Either the vma flag shouldn't get dropped or we should un-mark >> the PTEs. >> >> Is the vma flag maybe getting dropped because of some weird interaction with >> UFFD_EVENT_REMAP? > > Right, I think we should do the latter. > > We need to drop the vma flag by default, as you quoted in the other patch > in 2018, as the monitor process may not be able to process this otherwise, > seeing unknown address reported when read(). So instead we should drop the > uffd-wp bit here.. > > The patch should be straightforward, I'll test it a bit. One trick here is > pte_clear() won't work for clearing pte markers: I remember there's one > arch that will crash if pte_clear() on a temporary pte_t, which is exactly > this case.. so to avoid hitting that (again..), I think we'll need to open > code this just to be able to skip set_pte_at() by one "continue". > > I'll share the patch shortly. The other bit that made me scratch my head is that after the mremap, the VMA that contains the first 2 pages is no longer mlocked. I could just about be convinced this is ok for the first page (which was the one remapped), since the docs say any access to this will always fault (given MREMAP_DONTUNMAP). But that can't be right for the second page can it? We previously called mlockall(MCL_FUTURE|MCL_CURRENT) so the process expects to be able to access that page without delay. Is there a bug here too? > > Thanks, >
